Ever since the field of galvanometer-based optical scanning became established approximately 30 years ago, this type of scanning has been accomplished in substantially the same way. A mirror is attached to the shaft of a limited rotation motor, often referred to in the art as a galvanometer or “galvo.” As the shaft of the motor rotates, this in turn rotates the mirror. A light beam is reflected off the mirror and, as the mirror is rotated by the motor, the light beam is scanned. Often, two motors and two mirrors are used, and are arranged in such a way as to accomplish X-Y scanning. This is very well known in the art and can be found in numerous patents including Tanaka et al. U.S. Pat. No. 5,130,838 and Montagu U.S. Pat. No. 5,084,904.
The first limited rotation motors (galvanometers) to garner widespread use in optical scanning were made by General Scanning (Brosens U.S. Pat. No. 4,135,119) and MFE Corporation (Burke, Jr. U.S. Pat. No. Re. 31,062). These were of the “moving iron” variety—named after the fact that the rotor is made from a solid or laminated piece of iron. Moving iron galvanometer scanners provided very respectable performance and were in use for approximately 20 years, before being replaced by more modern scanner types. Disadvantages of moving iron scanners are their limited torque to inertia ratio, and relatively high electrical inductance, caused by the fact that the coil is completely surrounded by an iron magnetic circuit.
From approximately 1986 onward, the company Cambridge Technology, of Cambridge, Mass., has manufactured moving coil galvanometer scanners—named after the fact that the rotor is a moving coil of wire rather than iron. Moving coil galvanometer scanners have a number of advantages. One of the advantages is that they have a very low electrical inductance, often being an order of magnitude lower than moving iron counterparts. Another advantage is that the torque-to-inertia ratio is far greater than their moving iron counterparts. However, there are a few disadvantages to moving coil scanners that have prevented their widespread use. One disadvantage is that current flowing through the coil to create the scanning motion causes the coil to get hot, yet there is no effective way to quickly dissipate the heat that is generated because the coil is essentially floating in mid air. Because of this problem, moving coil scanners cannot be used for applications that require a high duty cycle (i.e. constant, high frequency scanning). Another disadvantage is that the long, unsupported spans of coil wires lead to multiple low-frequency torsional resonances, which are highly undesirable.
From approximately 1992 to the present, several companies have manufactured moving magnet galvanometer scanners—named after the fact that the rotor consists almost entirely of a magnet. Examples of moving magnet galvanometer scanners are described in various patents by Montague, including U.S. Pat. No. 5,424,632 and U.S. Pat. No. 5,936,324. Moving magnet scanners have some of the benefits of moving coil scanners, such as relatively low inductance and relatively high torque to inertia ratio. In addition, since the coil is mounted directly to the scanner's back iron, heat dissipation is more efficient than in moving coil scanners. However, there is still room for improvement in optical scanning, particularly in the realm of eliminating torsional resonances that exist largely because of the fact that a relatively long rotor is driving one end of a relatively wide mirror, often times through a shaft whose diameter is relatively small.
As the field of optical scanning has continued to develop, incremental improvements have been made to overcome barriers to higher performance and faster scanning speeds. For example, when moving iron scanners were being used, the barrier to higher performance was the limited torque to inertia ratio and the relatively high inductance. The development of moving coil and moving magnet scanners has overcome those two problems but, currently, torsional resonances present the most difficult barrier to achieving higher accuracy and faster scanning speeds.
Since the current state of the art continues to rely on the “mirror on the end of a motor shaft” construction, there is a “wind up” effect that occurs due to the torsional spring characteristic of long scanner rotors coupled with the mass and inertia of the mirror. Other torsional resonances also occur due to the shape of the mirror and the manner in which the mirror is attached to the shaft. In all cases, these torsional resonances exist because the mirror is being driven from only one end, and because of the torsional springiness of the mirror and motor components.
In response to these problems, various techniques have been employed in an attempt to decrease the frequency of these torsional resonances, or to otherwise manage them, including novel rotor designs described by Montagu in U.S. Pat. No. 5,424,632 and by Brown et al. in U.S. Pat. No. 6,433,449, and also by shaping the mirror and the mirror mount as described by Stukalin et al. in U.S. Pat. No. 6,243,188. Nevertheless, despite these improvements, torsional resonances remain a significant problem.
Torsional resonances continue to be a problem since, most often, galvanometer scanners are used in combination with closed loop servo drivers. These servo drivers are disturbed by the presence of resonances, and if the servo gain is set sufficiently high to achieve fast scanning, uncontrolled oscillation is likely to occur. As a result, servo gain must be kept relatively low and the closed-loop, small-signal bandwidth (i.e. the speed of the scanning) can only be made a fraction of the lowest torsional resonant frequency. For example, many moving magnet scanning systems have torsional resonance frequencies around 8 kHz to 10 kHz, yet in order to maintain good quality scanning, the servo gain and thus closed-loop small-signal bandwidth cannot be made greater than approximately 2.5 kHz.
Yet another disadvantage to the “mirror on the end of a motor” construction is that a substantial amount of the torque generated by the scanner, is actually expended in moving the scanner's rotor itself. Since the rotor ideally has the same inertia as the mirror, this means that effectively half the torque is wasted moving the rotor. This disadvantage is often overlooked as unavoidable, something that “comes with the territory” of placing a mirror on the end of a motor.
A careful review of performance characteristics of the three types of optical scanners mentioned above shows that the torque to inertia ratio is actually highest for moving coil scanners. Likewise, electrical inductance is lowest for moving coil scanners. These are two highly desirable characteristics. However, as noted above the two main drawbacks with moving coil scanners has been inability to effectively dissipate heat generated in the coil during scanning, and multiple low frequency resonances of the scanner caused by the long spars of the coil wires.